1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with fuel assemblies in the nuclear reactor core which each has a symmetric blanket assembly design composed of a number of centrally-located natural uranium fuel rods surrounded symmetrically by a greater number of peripherally-located enriched uranium fuel rods.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array of grids spaced along the fuel assembly length and attached to the guide thimbles. Top and bottom nozzles on opposite ends of the assembly are secured to the guide thimbles, which extend slightly above and below the ends of the fuel rods, so as to form the fuel assembly as an integral unit.
Each of the fuel rods in the array thereof in the fuel assembly includes nuclear fuel pellets which are responsible for creating the reactive power of the reactor core. All of the fuel assemblies of the reactor core are substantially identical except for different percent enrichment of fuel material in the fuel rods of one group of assemblies compared to another group. Traditionally, the fuel rods in some of the fuel assemblies of the core contain enriched uranium, generally known as fissile material, which is capable of sustaining a chain reaction, whereas the fuel rods in other of the fuel assemblies contain natural uranium, generally known as fertile material, which is incapable of sustaining a chain reaction, but which later converts to fissionable material upon exposure to neutron bombardment. It is conventionally well-known that natural uranium refers to uranium found in nature which consists of 99.3% U-238, 0.7% of U-235 and minute traces of U-234, whereas enriched uranium refers to uranium in which the percentage of the fissionable isotope U-235 has been increased above the 0.7% contained in natural uranium.
Basically, fuel assemblies containing enriched uranium are known as seed assemblies, while those containing natural uranium as known as blanket assemblies. Various reactor core loading arrangements of seed and blanket assemblies are known in the art. Several of such arrangements are described and depicted in U.S. Patents to Sherman et al U.S. Pat. No. (3,158,543), Creagan U.S. Pat. No. (3,211,621), Barry U.S. Pat. No. (4,096,033), Hill U.S. Pat. No. (4,326,919) and Uchikawa et al U.S. Pat. No. (4,378,329).
Many arrangements in use at the present time incorporate what is known as a radial blanket concept. This design concept positions fuel assemblies containing natural uranium, i.e. the blanket assemblies, directly on the core periphery, next to the core baffle, to improve fertile material conversion and decrease neutron leakage. However, such positional relationship of the blanket assemblies relative to the center and periphery of the core has two major drawbacks both of which are related to the asymmetric loading of natural uranium entailed in such arrangement. First, the enriched uranium of the seed assemblies, located next to the natural uranium of the blanket assemblies, is under-utilized and thus achieves lower than average burnup rates. Second, serious peaking problems can occur when these assemblies are shuffled to other locations in the core in which natural uranium is not along a core boundary.
Consequently, a need exists for improvement of the arrangement of natural and enriched uranium materials within the core of a reactor in a manner which increases the efficiency of the nuclear fuel while keeping potential leakage and peaking problems under control.